If one side of a triangle is extended, then the exterior
angle is greater than either of the opposite interior angles.

Let ABC be a triangle, and let one side of it BC be extended to D;
then the exterior angle ACD is greater than either of the
opposite interior angles, ABC, CAB.

(We now draw triangle BFC in such a way that triangles BAE, ECF will be congruent; that makes angle BAE equal to angle ECF. Eventually, Proposition 27, that will imply that the lines BA, CF are parallel.)